Natural lithium consists of two isotopes, 6Li and 7Li, with respective abundances of 7.52% and 92.48%. 6Li isotope has high ability to capture slow-moving neutrons. When a compound synthesized with isotope 6Li and deuterium, lithium deuteride, is bombarded by neutrons, a strong thermonuclear reaction occurs with a large amount of energy generation. 7Li isotope with a tiny thermal neutron absorption cross section (0.037b), can be used as a reactor core coolant for nuclear fusion reactor and a heat-carrying agent for thermal conduction. Thus, it can be seen that lithium isotopes, 6Li and 7Li, respectively have very important applications in nuclear energy. The separation and production of high purity lithium isotopes are related to the security of national energy and the implementation of sustainable development strategies.
Throughout the domestic and overseas methods by which 6Li and 7Li are separated from natural lithium, they can be roughly classified into chemical methods and physical methods. Chemical methods include lithium-mercury exchange, ion exchange chromatography, extraction, fractional crystallization, fractional precipitation and the like; physical methods include electromagnetic method, molten salt electrolysis, electron mobility, molecular distillation, laser separation and the like. Currently, only lithium-mercury method has been used for industrial production, due to its advantage in separation coefficient of lithium isotopes, basically around 1.05; in high exchange rate, with only a few seconds of half time of exchange under severe countercurrent condition; and in that two convective phases are prone to be formed in the system, being useful for the design and cascading of processes. However, there is also a great disadvantage in lithium-mercury method for separating lithium isotopes, that is to say, a large amount of mercury used in separation process will cause environmental and safety problems. Factories using lithium-mercury method to separate lithium isotopes in European and American countries have been partially closed; various countries are actively and secretly searching for a green and efficient method for separating lithium isotopes.
Since it has been found that a crown ether can selectively form a complex with a metal ion, especially an alkali metal ion, in accordance with the size of ring, related research on lithium isotope separation are rapidly carried out. Since 7Li and 6Li have different ionic radii, as well as their different densities of surface charge, there is a difference in adsorption capacity of crown ether between them. Based on this fact, different systems are formed for crown ethers to separate lithium isotopes, mainly including crown ether liquid-liquid extraction and crown ether resin chromatography. The crown ether extraction is a chemical exchange method which uses a crown ether as a neutral chelating extractant, wherein the extraction and separation of lithium isotopes need to undergo three steps, namely extraction, exchange and counter-extraction. The extraction and counter-extraction steps achieve the phase-transfer and counter-flow of materials, while the exchange step achieves the exchange and enrichment of isotopes. The equation of lithium isotope exchange can be expressed as:6Li(watery)++7Li(organic)7Li(water)++6Li(organic)
From Oct. 30 to Nov. 2, 1979, in the conference held in the United States on separation science and separation technology for energy application, Jepson from Monsanto made a report on chemical exchange of lithium isotopes by using macrocyclic polyethers. The author determined the separation factors of single-stage equilibria in two-phase chemical exchanges for enriching lithium isotopes in several systems. In the systems studied, 6Li are all enriched in the organic phases, the separation factors of single-stage equilibria vary from a minimum value of α=1.0086±0.0023 to a maximum value of α=1.041±0.006. This result indicates that Li-cryptand (2.2.1) systems have isotopic separation factors close to lithium-mercury systems, and crown ether systems have great flexibilities in complexing ligands, solvents and counterions. Therefore, such systems provide a new clue for the separation of lithium isotopes. The research on benzo-15-crown-5 derivative system is reported in Shengqiang Fang, Li'an Fu, et. al., Journal of isotopes, 1994, 7(3):168-170, and it is proposed that factors affecting the lithium isotopic separation effect in crown ether system include the inner structure of crown ether, the side group of crown ether, the concentration of crown ether, the organic solvent, the anion of lithium salt, the concentration of lithium salt and the temperature. Thus it can be seen that the separation of lithium isotopes by crown ether has a good separation effect; just regarding the separation factor, it is the most promising method for lithium isotopic separation. However, the aforesaid crown ether extraction system mainly uses small molecule crown ethers, hence there would be some problems resulted from such small molecule crown ethers in the application process, such as difficulties in counter-extraction and re-use, expensive prices, some environment pollutions caused by the usage of a large amounts of organic solvents and the like. A good way to solve the above problems is to achieve the loading of crown ether compounds on polymers as carriers.
Disclosed in CN1186506A is a preparation method of water-soluble polymer, which relates to a water-soluble polymer wherein polyvinyl alcohol is grafted with 15-crown-5 ether, but such material is not intended for lithium isotopic separation, and the grafted structure 15-crown-5 does not contain a benzene ring as electron-donating group, thus it has a relatively weak ability of lithium isotopic separation. Disclosed in U.S. Pat. No. 4,600,566A (2006) is a lithium isotope separation method by chromatographic separation based on cryptand-grafted styrene-divinylbenzene resin. This method involves cryptand resin in lithium isotope separation process between solid-liquid two phases, since water molecules are not present in the system, the lithium ion-hydration effect is eliminated, the single-stage separation factor of resin reaches 1.03-1.06, lithium-6 is enriched in resin phase. However, the high energy in complexing of cryptand with lithium ions causes a new problem for desorption of lithium ions, which limits the reusability of resin material. It is reported in Kim et. al., Journal of Radioanalytical and Nuclear Chemistry, 2000, 245:571-574 that amino-benzo-15-crown-5 grafted Merrifield peptide resin is used to separate lithium isotopes. The method results in enriched lithium-7 in resin phase, reaching a separation factor of 1.026. However, since the particle size of resin is too small (50-100 μm), the adsorption and desorption of isotopes has a low efficiency.